A method is known for example from the patent specification DE 198 54 939 C2. In accordance with the patent specification, the rest phase of a beating heart is established here by an automatic analysis of the measurement data of a single-source CT apparatus and comparison of the complementary projections in parallel geometry. In this case, complementary projections are understood to mean projections offset by an angle of 180°, such that each beam of these projections has a complementary partner having the same path but in the opposite direction.
In the case of unmoving objects, it can be assumed to a first approximation that the difference between the complementary beams of the complementary projections is equal to 0 apart from negligible noise contributions. In the case of moving objects or partly moving objects, the deviation of the difference from 0 which goes beyond the noise contribution is a measure of the movement of the measured object during half a revolution time of the CT scanner. In this case, the sum of the absolute deviations of the complementary parallel projections can be regarded as a suitable coefficient of measure σn.
This measure of movement σn can be plotted as a function of the projection angles and thus as a function of time. If the measure of movement σn lies below a specific threshold value, then it is assumed that the heart was in the rest phase in the time between the measurement of the two complementary projections.
This method has some disadvantages. Firstly, the time difference between the complementary projections corresponds to half a rotation time, that is to say according to the current prior art not less than 0.33 s/2=165 ms. That is relatively long compared with typical time constants of heart movement. Under certain circumstances, therefore, all movements which take place within this time window of 165 ms cannot be registered or can be registered incorrectly.
Secondly, the method is based on the analysis of parallel projections. Present-day CT apparatuses record fan projections, however, from which the parallel projections first have to be generated by interpolation. A parallel projection is then composed of measured values of fan projections measured at different instances, such that the assignment of a specific recording time to a parallel projection is difficult and can relate approximately only to the rotation center.